The manufacture of semiconductor devices involves the performance of a series of process steps using a variety of high tech production and metrology tools in a certain order and often within a certain period of time. The primary function of a wafer logistics system in a wafer fabrication facility, or “fab,” is to deliver the wafers to each of the tools at the right time, as well as to track the location and status of the wafers throughout the process.
Automated material handling systems (“AMHS”) are applied to wafer fabs to carry out the automated functions more efficiently, consistently, and safely than can be done via manual means. While growth in wafer size from 200 mm to 300 mm has rendered the fabrication process more economical in some respects, it has also placed additional demands on the process. Such demands include the necessity for cross-floor and cross-phase transportation and increased transportation volume, the combination of which often results in traffic jams. Additionally, the investment in the AMHS hardware is large.
300 mm process tools can be divided into two main categories, including fixed buffer process tools and internal buffer process tools. A fixed buffer process tool has only fixed load ports for receiving wafer carriers, such as front opening unified pods (“FOUPs”) and no internal buffer for carrier storage; consequently, wafers are loaded and unloaded directly from a carrier transported via an overhead transport (“OHT”) system at the load ports for processing by a fixed buffer process tool. In contrast, an internal buffer process tool includes locations within the tool in which to store carriers, each of which contains a portion of a batch, or a “lot”, prior or subsequent to processing the wafers, not including the load ports. Internal buffer tools are thus capable of implementing continuous batch processing. For example, assuming a first batch comprising one or more lots is being processed by a process unit of the tool, a second batch comprising one or more lots can be stored in the buffer thereof. Subsequently, the AMHS determines whether one of the load ports of the tool is available and, if so, sends a carrier containing a lot comprising a portion of a third batch to the tool. This may result in a conflict, however, if by the time the carrier containing the third batch lot arrives at the previously available load port, a carrier containing a lot of the second batch is unloading from the buffer via the load port, causing a conflict at the load ports. Upon the occurrence of such a conflict, the carrier containing the third batch lot will be returned to the location from which it originated, as unloading a carrier from a tool generally has a higher priority than loading a carrier to a tool. Currently, it is not possible to determine whether such a conflict will occur until the carrier containing the third batch lot arrives at the tool via the OHT system.